[Objectives] Menstrual cycle monitoring is an effective method for the breeding management in ex-situ conservation of non-human primates. Based on analyses of fecal sex steroid hormone of a female Yellow-cheeked Gibbon Nomascus gabriellae, the duration and regularity of its menstrual cycles were determined, and attempts to assess whether females have reproductive disorders from a reproductive endocrine perspective were made. Androgens were used as indicators to explore whether males have reproductive physiological disorders. The study helps to provide data support for the natural breeding of captive Yellow-cheeked Gibbons and the application of artificial assisted reproduction technology in the future. [Methods] From April 2016 to April 2017, fecal samples from a pair of Yellow-cheeked Gibbon were collected every 1﹣3 d non-invasively. These samples were further analyzed for estrone-3- glucuronide (E1G), progesterone (P4) and testosterone (TEST) levels by ELISA. The coefficient of variation CV for hormone determinations was < 5% within batches and < 10% between batches. The data were sorted by Excel and analyzed by SPSS 22.0 with normal distribution and homogeneity of variance test. One-way ANOVA was used for significance analysis for sampling that conformed to normal distribution, nonparametric tests Mann-Whitney U test was used for those that did not conform to normal distribution. The correlation between female estrogen and male testosterone was analyzed using the Pearson’s method of analysis (two-tailed test). The significance level was set at 0.05. [Results] Estrogens and progesterones were synchronized during menstrual cycles (Fig. 1). More precisely, at the start of oestrum, estrogen level began to increase from a baseline, after reaching the first peak, dropped dramatically, and followed by ovulation, which signifying the end of the follicular phase. Progesterones remained at a base level during the follicular phase. After ovulation, both progesterones and estrogens shared an ascending pattern, and then decreased to base levels after reaching the peak values simultaneously. A total of 11 complete menstrual cycles of the female “Vivi” were found during the study period (Fig. 2), and the average length of menstrual cycles was 23.3 ± 3.1 d (21﹣31 d) (Table 1), which was not significantly different from that of the menstrual cycles based on the estrogen (E1G) bimodal interval (21.3 ± 3.8) d (Z =﹣1.562, P = 0.133), nor from that of the progesterone (P4) pattern 20.9 ± 2.2 d (Z =﹣1.693, P = 0.101). The length of the follicular phase in females was 7.9 ± 2.4 d, and the luteal phase was 13.4 ± 2.8 d. TEST levels in male feces did not show significant cyclic variation (Fig. 3). The correlation between male (TEST) and female (E1G) during the first 10 menstrual cycles was analyzed, and there was no correlation between these two parameters (r = 0.139, P = 0.097). [Conclusion] The results showed that the Yellow-cheeked Gibbon had a similar menstrual cycle to other primates, and the menstrual cycle of this female individual was relatively obvious and regular. Based on the changing pattern of estrogen during the follicular phase, it is possible to prejudge the climax of estrus in females and guide the mating of Yellow-cheeked Gibbons. It is also possible to make a preliminary diagnosis of whether a female is pregnant or not based on the level of progesterone during the luteal phase. The sex hormone index of menstrual regularity can be an aid in the diagnosis of infertility in the Yellow-cheeked Gibbon and even in other primates.